1,2,4,5,6,7,7,7,- 1The University of Tokyo, Japan (shohei.aoki@edu.k.u-tokyo.ac.jp)
- 2Kyoto Sangyo University, Japan
- 3National Astronomical Observatory of Japan, Japan
- 4Tohoku University, Japan
- 5NASA Goddard Space Flight Center, USA
- 6Università degli Studi della Basilicata, Italy
- 7Royal Belgian Institute for Space Aeronomy, Belgium
Recent observations by NASA’s Curiosity Rover have revealed unexpected seasonal changes in molecular oxygen (O2) levels in the Martian atmosphere at Gale Crater, its landing site [1]. This finding is puzzling, as the atmospheric lifetime of O2 is expected to be much longer—at least 10 years [2]. Although O2 is generally stable, its volume mixing ratio is influenced by the seasonal sublimation and condensation of CO2 between the polar ice caps and the atmosphere. However, the observed O2 variability is significantly greater than predicted, with increases of up to 30%. These changes peak in late spring and early summer and decline in mid-winter, exceeding the variability that can be explained by known chemical processes.
O2 was first detected in the Martian atmosphere through ground-based observations in 1971–1972 [3,4] and again in 1982 [5]. Using high spectral resolution (R > 100,000) instruments in the visible range on 2–3 meter-class telescopes, faint O2 spectral lines were detected near 762 nm. These measurements yielded a globally averaged O2 mixing ratio of 1300 (±130) ppm. Later observations using the Herschel Space Telescope in the sub-millimeter range confirmed a similar average of 1400 (±200) ppm [6]. However, these earlier studies provided only disk-averaged values and did not resolve spatial distribution, and the measurement uncertainties remained relatively large.
In this study, we report new measurements of Martian oxygen using the High Dispersion Spectrograph (HDS) on the Subaru Telescope. Subaru/HDS offers high spectral resolving power (R > 100,000) in the visible range and features a long slit (~14"), which enables spatially resolved measurements of O2. This allows us to map the distribution of oxygen across latitude, longitude, and local time—helping to determine whether the seasonal O2 variations observed by Curiosity are global or localized. An additional advantage of Subaru/HDS is its wide spectral coverage, enabling simultaneous observation of ~13 O2 lines between 762–765 nm. This improves the accuracy of the retrieved O2 mixing ratios and enables a more detailed analysis of seasonal variability. Furthermore, CO2 lines at 869 nm—whose strengths are comparable to those of O2—can be used to estimate the effective airmass and assess the influence of atmospheric aerosols.
Observations were conducted on 15 February and 11 April 2025, corresponding to solar longitudes (Ls) of 44° and 68° in Martian Year 38. The latter coincides with the O2 enrichment period reported by Curiosity, while the former precedes it. The angular diameters of Mars during the observations were 12.3" and 7.6", respectively. The Doppler shifts between Mars and Earth were 11 km/s and 16 km/s, sufficient to separate Martian O2 lines from strong telluric O2 features. We present the resulting global O2 distribution maps and compare them with predictions from a Mars climate model.
References:
[1] Trainer et al. 2019, Journal of Geophys. Res.: Planets, 124, 3000.
[2] Lefèvre & Krasnopolsky, 2017, Cambridge University Press.
[3] Barker, E. S. 1972, Nature, 238, 447.
[4] Carleton, N. P., & Traub, W. A. 1972, Science, 177, 988.
[5] Trauger & Lunine, 1983, Icarus, 55, 272.
[6] Hartogh et al., 2010, Astron. & Astro., 521, L49.
How to cite: Aoki, S., Sagawa, H., Arai, A., Kagitani, M., Villanueva, G., Faggi, S., Liuzzi, G., Daerden, F., Viscardy, S., Robert, S., and Vandaele, A.: Global mapping of molecular oxygen on Mars, EPSC-DPS Joint Meeting 2025, Helsinki, Finland, 7–12 Sep 2025, EPSC-DPS2025-430, https://doi.org/10.5194/epsc-dps2025-430, 2025.
